1. Field of the Invention
The present invention relates to transparent coating systems that are applied to transparent substrates, such as those used as windows in aircraft and military ground vehicles, to improve their environmental durability.
2. Description of Related Art
Aircraft and military ground vehicles are often required to operate under temperature extremes and other adverse environmental conditions. The external surfaces of their windows, whether made from glass or plastic, tend to become pitted due to the erosive effects of rain, hail, sand, dust, and the like. Such effects cause a reduction in visibility. Some vehicle windows also include transparent thin film coatings which have been applied to their external surfaces. They also are vulnerable to the same type of damage from these erosive effects, which leads to a loss of their functionality as well.
Tough, thermoset polyurethane coatings and films, sometimes referred to as "liners," exemplify one type of external transparent coating that has been used to protect glass and plastic surfaces from particle erosion and abrasion. While such polyurethanes are effective in reducing erosion damage, such coatings are not durable enough to withstand other adverse environmental conditions. For example, glass substrates covered with an erosion resistant polyurethane coating tend to fracture or spall when exposed to temperature extremes due to differences in the coefficient of thermal expansion between the polyurethane and the glass. Thermal expansion and contraction can also cause fracture or delamination of thin film coatings on glass or plastic substrates. Known polyurethane coatings also tend to be degraded by other environmental effects, such as UV exposure and the ingress of water, which results in loss of adhesion at one or more of the coating interfaces, ultimately leading to delamination.
Accordingly, the need exists for a transparent coating system that, when applied to a transparent substrate, will improve its environmental durability.